CREATE program

Warm bottom does not imply ice sheet collapse.

Why would anyone care about the temperatures at the base of an ice sheet during the last ice age? Presently, there are two ice sheets on Earth, the Greenland and Antarctic Ice Sheet. Due to increasing global temperatures, scientists are plagued with questions concerning their fate. Over the past two decades, observations have revealed a significant mass loss from the Greenland Ice Sheet, while studies warn of the potential collapse of an important component holding together the West Antarctic Ice Sheet. The collapse, alone, of the West Antarctic Ice Sheet would lead to a rise in sea level by 3 m. However, to fully understand the effects of future climate change on ice sheets, it is necessary to understand their growth, decay and collapse. This is why we look to the past.

During the last glacial cycle, ~120000-12000 years before present, large ice sheets formed in the Northern Hemisphere, including the Laurentide Ice Sheet, which kept a great part of Canada under more than 1 km of ice. These ice sheets are governed by ice dynamics, that is the balance between snow accumulation and its own weight, and their interactions with the climate. Since we cannot travel back in time to study them, indicators of past climate are necessary tools. To study the ice dynamics of the Laurentide Ice Sheet, we used borehole temperature-depth profiles to reconstruct ground surface temperature histories and the temperatures at the base of the ice sheet.

It has been known for a long-time that as you go deeper within the Earth, the temperature increases. If there are no changes in ground surface temperature, it is assumed that this temperature-depth profile depends on the outflow of heat from Earth’s interior, which for climate purposes is constant. However, when there are persistent increases in ground surface temperature, the extra heat propagates into the subsurface leaving a record as perturbations to the otherwise unperturbed thermal regime underground. As early as the 1930s, scientists have been inferring past climate from these perturbed underground temperatures. It was only in the 1980s, due to concerns about increasing global temperatures, that this method became widespread. Most of these studies have focused on reconstructing the climate for the last 1000 years from relatively shallow (~500 m) boreholes. But, long-term persistent variations in surface temperature affect temperature in the subsurface to great depths. This allows for the reconstruction of the ground surface temperature history for the last glacial cycle and the determination of the temperatures at the base of the ice sheet.

In a recent paper published in the European Geosciences Union journal Climate of the Past, PhD student Carolyne Pickler (StFX’s NSERC CREATE program and Université du Québec à Montréal), Dr. Hugo Beltrami (St. Francis Xavier University) and Dr. Jean-Claude Mareschal (Université du Québec à Montréal.), analyzed thirteen temperature profiles from deep boreholes (≥1500 m deep) in eastern and central Canada, a region covered by the southern portion of the Laurentide Ice Sheet, to reconstruct the ground surface temperature histories during and after the last glacial cycle. They estimate basal temperatures between -1.4 and 3.00C throughout the last glacial cycle. These temperatures are near the melting point of ice, allowing for the melt and flow of water at the base of the ice sheet. Ice sheet basal temperatures are important parameters used in models of ice dynamics and suggest that it is possible to transport large quantities of water from the interior of the ice sheet, leading to a thin, climatically vulnerable ice sheet. However, as Pickler’s team points out, despite melting at the base, the ice sheet persisted for more than 30000 years prior to its collapse. This suggests that basal temperatures near the melting point do not necessarily imply instability in an ice sheet. However, combined with other processes they could lead to instability and collapse. Scientists have shown that the present-day ice sheets have basal temperatures near the melting point of ice. While Pickler et al. work has shown that this alone may not be cause for concern; combined with other conditions it could result in ice sheet instability. The group’s research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grants and CREATE programs.

Figure Caption: Ice thickness of the Laurentide ice sheet 20000 years ago as modelled from a glacial system model (Tarasov et al., 2012)